LED Emergency Lamp

Make this simple  White LED emergency lamp using few components. It can be plugged into the AC socket and it turns on at the moment when power failures. It emits cool white light in the room which is sufficient even for reading purpose. It is powered by a 4.5 volt rechargeable battery and uses 1 watt White LED.

Something about 1 watt white LED
It is now widely used in lighting applications and emergency lights. No semiconductor emits pure white light. Actually the white light is a feeling created by the mixing of the primary colours, the red green and blue. The semiconductor material in the white LED is the Indium gallium nitride blue chip coated with phosphor. The phosphor has the property to emit white light when energised by blue or ultra violet light.

The blue chip in the white LED is coated with a converter material. When a potential difference is applied between the anode and cathode of LED, the blue chip emits blue light. The converter material then emits yellow light. The mixture of this yellow light and blue light is perceived by the eyes as white light. The light from the white LED has both peaks in the blue and yellow region but human eye will identify the peaks as white light. Ordinary white LED requires 3 volts and around 40 mA current to give maximum brightness. The High watt LEDs requires 3 volts and 100 to 300 mA current.

About the Circuit
The circuit uses a small 4.5 volts 300 mA stepdown transformer, a bridge rectifier comprising D1 through D4 and the smoothing capacitor C1 as the power supply to charge the 4.5 volts rechargeable battery. The 4.5 volt transformer is readily available and is generally used in LED emergency lights. It drops the 230 volts AC to low volt AC which is then rectified by the full wave rectifier. Capacitor C1 makes the DC ripple free for charging. Resistor R1 provides around 80 milli ampere current for charging. Since the circuit is always plugged in, low current is ideal for charging. The rechargeable battery used in the circuit is a 4.5 volts  battery.

When the mains power is available, battery charges via R1 and D5. At the same time, the PNP transistor gets positive bias through R1 and it remains off. When the power fails, D5 reverse biases and the base of T1 will become negative. It then conducts and light the LED. When the power resumes, LED automatically turns off.

How to Set?
The circuit can be assembled on a small piece of perf board. Enclose the circuit, transformer and the battery in a plug in type adapter box. Fix LED outside the box preferably with a reflective back ground. You can paste Foil paper for this purpose. Since high current flows through T1 when the LED lights, T1 requires a heat sink. 1 Watt white LED cost around Rs.50 and the battery Rs.150.